Epidemiological, clinical and preclinical studies have shown that nonsteroidal anti-inflammatory drugs (NSAIDs) possess striking cancer chemopreventive activities. However, toxicities related to cyclooxygenase (COX) inhibition and the suppression of physiologically important prostaglandins limit the regular use of NSAIDs by humans for cancer chemoprevention. Previous studies have demonstrated that the antineoplastic activities of NSAIDs can be attributed to a COX-independent mechanism that induces apoptosis. Thus, there is strong evidence to support the development of new drugs that have the potential to be safer and more efficacious for cancer chemoprevention. Recent preliminary observations have prompted us to hypothesize that specific microRNAs (miRNAs) can mediate the cancer chemopreventive activity of the NSAID sulindac sulfide (SS). MiRNAs are small, non-coding RNA molecules that play important roles in controlling cell growth, differentiation, apoptosis, and tumorigenesis through the translational repression of their cognate target genes. Our pilot results showed that SS treatment of human breast cancer cells up-regulated 58 and down-regulated 59 miRNAs when a 2-fold cutoff was used. One miRNA, termed miR-200b, exhibited a pronounced induction in response to treatment with SS and a more active, non-COX-inhibitory SS derivative, sulindac sulfide amide (SSA), which we recently found to inhibit mammary tumorigenesis in the DMBA rat model. MiR-200b can reduce the expression of the cGMP-specific phosphodiesterase PDE5, which has been previously reported to be overexpressed in tumor cells and closely associated with the apoptosis-inducing activity of SS. These results suggest that miR-200b is involved in a key molecular event that mediates the cancer chemopreventive activity of sulindac. Moreover, we found that miRNAs could mediate the suppression of breast cancer cell invasion by SS through a mechanism involving NF-:B transcriptional regulation. The specific aims to address this hypothesis are as follows: 1) determine if miR-200b mediates the sensitivity of breast tumor cells to SS and SSA by suppressing PDE5 and 2) determine if the inhibitory effects of SS and SSA on tumor cell invasion are mediated by miRNA and NF-:B inhibition. The proposed studies are anticipated to provide valuable insights into how specific miRNAs mediate the breast cancer chemopreventive activity of sulindac, and these studies will explore new activities involving the suppression of tumor cell invasion.